In numerous industries, and in particular in the field of volumetric fluid metering, there exists a need to determine a physical parameter represented by variation in time of a physical magnitude G, said physical parameter representing, for example, the integral of a physical magnitude such as the flow rate of a fluid, in which case the parameter corresponds to the volume of fluid.
Certain measurement systems rely on the principle whereby direct access is provided to the integral of the physical magnitude, but not to the instantaneous value of said physical magnitude.
However, such systems are being ever more frequently modified for the purpose of integrating new functions that require knowledge of the instantaneous value of the physical magnitude, and to do this, appropriate electronic means are added thereto, specifically for the purpose of converting analog signals into digital signals and for performing processing on said digital signals.
Thus, in modified measurement systems, the physical parameter representing the integral V of a physical magnitude G is no longer directly obtained, but is determined by a method of the following type: at instant t, the physical magnitude G representative of a certain state is measured and processed by a data acquisition and processing system which may, for example, comprise one or more sensors together with means for processing the signal delivered by the sensor(s) (amplifier, analog-to-digital converter, . . . ), thereby providing an estimated value G.sub.m (t) of the physical magnitude G at instant t, which value is also referred to as an "estimator". Digital processing then makes it possible to determine the physical magnitude, i.e. the time integral V.sub.m of the estimator G.sub.m at instant t,V.sub.m (t)=V.sub.m (t-1)+G.sub.m (t).DELTA.t where V.sub.m (t-1) represents the time integral of the estimator G.sub.m at instant (t-1), and where .DELTA.t represents the fixed time interval between two successive instants at which the integral V.sub.m of the estimator G.sub.m has been determined. After the integral V.sub.m has been computed and stored, the measurement system waits for a period .DELTA.t before estimating a new value of the physical magnitude G. The steps mentioned above are then repeated over time.
Unfortunately, the Applicant has observed that variations in time of the physical magnitude G are separated by periods during which the magnitude G varies little or not at all.
The above considerations lead to adapting the time interval between two instants at which the physical magnitude G is measured to variations in time of said physical magnitude G in application of the following principle:
when the physical magnitude G is varying little or not at all, then measurement instants are spaced out; whereas
when the physical magnitude G is varying to a greater extent, then measurements are taken at more frequent intervals.
Patent EP 0 019 672 discloses a method of sampling various physical magnitudes (temperature, pressure, flow rate, . . . ) while samples are being taken by a self-contained tool lowered down an oil well. That method provides for taking measurements of the physical magnitudes concerned at sampling time intervals that are variable so as to optimize tracking of the variations in said physical magnitudes, and for processing the measured data, thereby making more efficient use of a data storage memory that has limited storage capacity.
However, while determining the physical parameter representative of the integral of the physical magnitude G in discrete time, the measured instantaneous value or estimator G.sub.m of said physical magnitude is obtained with a certain amount of error that depends on the specific way in which measurement and/or estimation is done.
Furthermore, the determination of the physical parameter representative of the time integral V.sub.m of the measured physical magnitude G.sub.m is itself generally subject to error, given that it is not possible to know accurately what variations have taken place in the physical magnitude between two measurement instants.
The Applicant has observed that in numerous technical fields, such as the field of metering, where accurate measurements are required and where measurement instants must be as widely spaced apart as possible, e.g. because of problems of data storage capacity or problems of electricity consumption by the sensors and the associated electronic circuits which are powered by one or more electrical batteries of limited capacity, it can be essential to approximate in discrete time and as accurately as possible a physical magnitude G that is capable of varying continuously in time while making sure that a certain predetermined error threshold is not exceeded in the evaluation of the time integral of the measurement of said physical magnitude.